As optical fibers that transmit optical signals, single mode fiber (SMF) and dispersion shifted fiber (DSF) are cited, for example. For example, in Japan, a transmission line is not necessarily composed only of the SMF and possibly includes the DSF partly.
The DSF is designed in such a manner that the chromatic dispersion becomes zero. However, when a wavelength-multiplexed optical signal in the C-band (1.55 (nm) band) is transmitted to the DSF, signal distortion is caused due to the non-linear optical effect and the signal quality deteriorates. In contrast, in the SMF, the influence of the non-linear optical effect is small with a wavelength-multiplexed optical signal in either of the C-band and the L-band (1.59 (nm) band). Regarding the C-band and the L-band, there is a method in which the wavelength band of wavelength-multiplexed optical signals is converted between the C-band and the L-band.
For example when only a halfway section of a transmission path of wavelength-multiplexed optical signals is the DSF and the other sections are the SMF, the non-linear optical effect is suppressed if the power of the wavelength-multiplexed optical signal input to the DSF is sufficiently attenuated by an optical attenuator or the like. However, the power for reception of the wavelength-multiplexed optical signal is insufficient and therefore the transmission becomes difficult or sufficient transmission distance is not obtained. For this reason, in the above-described transmission path, optical transmission equipment compatible with the L-band is set at each node and wavelength-multiplexed optical signals in the L-band are transmitted.
For example, if a transmission path of wavelength-multiplexed optical signals is switched from the SMF to the DSF halfway, the deterioration of the signal quality is suppressed by converting the wavelength band of wavelength-multiplexed optical signals from the C-band to the L-band. In this case, in the section of the DSF, optical transmission equipment compatible with the L-band is set at each node and wavelength-multiplexed optical signals in the L-band are transmitted.
However, the optical transmission equipment compatible with the L-band has a small market size and therefore is expensive compared with optical transmission equipment compatible with the C-band. For example, a transponder that transmits and receives optical signals that are wavelength-multiplexed into a wavelength-multiplexed optical signal is set for each communication line and therefore the ratio of the configuration in the whole transmission system is high. Therefore, for example, when the number of transponders compatible with the L-band increases, the apparatus cost of the whole transmission system increases.
The following is a reference document.
[Document 1] Japanese Laid-open Patent Publication No. 2001-274750.